Next generation memory storage with a new block copolymer structure


June 28, 2023

(Nanowerk Highlights) Block copolymers – plastics capable of forming a variety of very small, structured patterns – are generating significant interest in the scientific community. With potential applications in next-generation computer chips, filters, and optical devices, the potential of this material looks promising. This promise is largely dependent on the arrangement and proportions of each type of polymer or building block within the block copolymer.

A new study, published in ACS Macro Letter (“Tetragonally Packed Inverted Cylindrical Microdomains of Binary Block Copolymer Blends with Enhanced Hydrogen Bonding”), have now devised a new method for forming high-density square patterns from these structures, paving the way for potential breakthroughs in high-density memory storage devices.

“Our findings are centered around a novel nanostructure, ‘inverted cylinder tetragonally packed’, which we achieve by fusing two block copolymers,” Jin Kon Kim, a professor at the Pohang University of Science and Technology and Program Director of the National Creativity Research Initiative for Smart Block Copolymers in Korea, who led the research, told Nanowerk. “The ‘reversed’ in the name refers to the principal component being cylindrical in the minor component matrix, an unusual characteristic considering that conventionally, the minor components are cylindrical.” Inverted cylinder with hexagonal (left below) and square (right below) cross-section by combining conventional (left above) and cylindrical (right above) lamellae Inverted cylinder with hexagonal (bottom left) and square (bottom right) cross-sections by combining conventional (top left) and cylindrical lamellae (top right). (Image: Prof. Kim, POSTECH)

These cylindrical nanostructures have been used in a wide range of applications, but certain limitations, such as the large distance between adjacent cylinders due to lower volume fraction, have been a barrier to fabricating high density nanopatterns. Hexagonal cylindrical packing is also not suitable for device processing compared to tetragonal packing.

Overcoming this challenge, the research team managed to fabricate an inverted cylindrical nanostructure with a large volume fraction and tetragonal packaging.

“In conventional cylindrical microdomains, the minor component with a volume fraction of less than 0.35 becomes cylindrical in the majority component matrix,” explained Kim. “However, in an inverted cylindrical microdomain, the principal component having a volume fraction greater than 0.5 becomes cylindrical in the minor component matrix.”

This innovation in cylinder shaping is critical to the production of high-density arrays, which are instrumental in creating the next generation of memory devices.

“Normally, when we use block copolymers to produce cylindrical nanostructures, the volume fraction of the cylinder is less than 0.35. This leads to large distances between neighboring cylinders, hindering the generation of high-density nano-patterning,” explains Kim.

In simpler terms, think of cylinders as trees in a forest – the smaller the volume fraction (less than 0.35), the fewer trees (cylinders) there are, resulting in a more diffuse forest. This is why it is difficult to fabricate dense, or high-density, nano-patterns.

Additionally, Kim notes, “Hexagonal cylinder packers, a typical result with lower volume fractions, are not as efficient at processing devices as tetragonal or square packers.”

In this study, the team increased the cylinder volume fraction to 0.7. This is like planting more trees in a forest—more trees (cylinders) allow them to get closer to each other, thus achieving a high density array. This opens up more possibilities for creating dense nano-patterns, which are important for developing ultra-compact memory storage devices.

This is an important breakthrough, as it pushes beyond conventional boundaries and offers the potential to develop ultra-high density memory devices. However, achieving this very high density pattern requires reducing the cylinder size, a feat that can be realized by using block copolymers with very small molecular weights. But as Kim notes, “With decreasing molecular weight, the two components in the block copolymer cannot be microphased separated to form a cylinder.”

The researchers believe this problem can be solved with certain block copolymers in which the two components exhibit a very high degree of mismatch.

As for the application, these nanostructures can be transferred as extremely high density cylindrical nanopattern arrays on the substrate. This can then be used as a template for ultra-high-density nanodot arrays, nano-contact holes.

Going forward, Kim sees a need for more diverse nanostructures and greater size control in the field. “To make extensive use of self-assembled nanostructures, this development can be realized by blending block copolymers with complex chain architectures and introducing very strong tensile interactions between blocks.”

Despite these challenges, recent advances and future potential of block copolymers in nanostructures and memory devices indicate a promising trajectory for this field of study.

Michael Berger

– Michael is the author of three books by the Royal Society of Chemistry:
Nano-Society: Pushing the Boundaries of Technology,
Nanotechnology: A Small Future And
Nanoengineering: Skills and Tools for Making Technology Invisible
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